α-SnWO 4 is an n-type metal oxide semiconductor that has recently attracted attention as a top absorber material in a D4-tandem device for highly efficient solar water splitting due to the combination of an ideal bandgap (∼1.9 eV) and a relatively negative photocurrent onset potential (∼0 V vs RHE). However, up to now, α-SnWO 4 photoanodes have not shown high photoconversion efficiencies for reasons that have not yet been fully elucidated. In this work, phasepure α-SnWO 4 films are successfully prepared by pulsed laser deposition. The favorable band alignment is confirmed, and key carrier transport properties, such as charge carrier mobility, lifetime, and diffusion length are reported for the first time. In addition, a hole-conducting NiO x layer is introduced to protect the surface of the α-SnWO 4 films from oxidation. The NiO x layer is found to increase the photocurrent for sulfite oxidation by a factor of ∼100, setting a new benchmark for the photocurrent and quantum efficiency of α-SnWO 4 . These results provide important insights into the photoelectrochemical properties and limitations of α-SnWO 4 and point toward new strategies to further improve the performance of this promising material.

Molybdenum sulfide MoS x is considered as attractive hydrogen evolution catalyst since it is free of noble metals and shows a low overvoltage. Especially, amorphous molybdenum sulfide has attracted attention because of its high catalytic activity. However, the catalytic mechanism of the hydrogen evolution reaction is not yet fully understood. Therefore in our study, layers of MoS x were deposited by reactive magnetron sputtering varying the substrate temperature in the range from room temperature (RT) to 500°C. The morphology and structure of the films change significantly as a function of temperature, from an amorphous to a highly textured 2H-MoS 2 phase. The highest catalytic activity was found for amorphous layers deposited at RT showing an overvoltage of 180 mV at a current density of-10 mAcm-2 in a 0.5 M sulfuric acid electrolyte (pH 0.3) after electrochemical activation. As detected by Raman spectroscopy the RT deposited catalyst consists of [Mo 3 S 13 ] 2and [Mo 3 S 12 ] 2entities which are interconnected via [S 2 ] 2and S 2ligands. When sweeping the potential from 0.2 to-0.3 V vs RHE a massive release of sulfur in form of gaseous H 2 S was observed in the first minutes as detected by differential electrochemical mass spectroscopy (DEMS). After electrochemical cycling for 10 min, the chains of these clusters transform into a layer-type MoS 2-x phase. In this transformation process, H 2 S formation gradually vanishes and H 2 evolution becomes dominant. The new phase is considered as a sulfur deficient molybdenum sulfide characterized by a high number of molybdenum atoms located at the edges of nano-sized MoS x islands, which act as catalytically active centers.

Cu doped NiO (Cu:NiO) back contact layers are inserted between FTO substrates and CuBi2O4 thin films to improve the performance of CuBi2O4 photocathodes.

Gallium oxide (Ga2O3) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) applying a capacitively coupled plasma source where trimethylgallium (TMGa) as the gallium precursor and oxygen (O2) plasma were used in a substrate temperature (Ts) in range of 80–200 °C. TMGa exhibits high vapor pressure and therefore facilitates deposition at lower substrate temperatures. The Ga2O3 films were characterized by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), and capacitance-voltage (C-V) measurements. The SE data show linear thickness evolution with a growth rate of ∼0.66 Å per cycle and inhomogeneity of ≤2% for all samples. The refractive index of the Ga2O3 thin films is 1.86 ± 0.01 (at 632.8 nm) and independent of temperature, whereas the bandgap slightly decreases from 4.68 eV at Ts of 80 °C to 4.57 eV at 200 °C. XPS analysis revealed ideal stoichiometric gallium to oxygen ratios of 2:3 for the Ga2O3 layers with the lowest carbon contribution of ∼10% for the sample prepared at 150 °C. The permittivity of the layers is 9.7 ± 0.2 (at 10 kHz). In addition, fixed and mobile oxide charge densities of 2–4 × 1012 and 1–2 × 1012 cm−2, respectively, were observed in the C-V characteristics. Moreover, the Ga2O3 films show breakdown fields in the range of 2.2–2.7 MV/cm. Excellent optical and electrical material properties are maintained even at low substrate temperatures as low as 80 °C. Hence, the TMGa/O2 PEALD process is suitable for electronic and optoelectronic applications where low-temperature growth is required.

Efficient charge transfer is achieved by coating WSe2 photocathodes with an earth abundant photocatalyst – ammonium thiomolybdate (NH4)2Mo3S13.

Cuprous oxide (Cu 2 O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu 2 O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu 2 O surface as well as the interface between Cu 2 O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu 2 O and provide guidance for future material development.

Ultra-thin perovskite absorber layers have attracted increasing interest since they are suitable for application in semi-transparent perovskite and tandem solar cells. In this study, size and density controlled plasmonic silver nanoparticles are successfully incorporated into ultra-thin perovskite solar cells through a low temperature spray chemical vapor deposition method. Incorporation of Ag nanoparticles leads to a significant enhancement of 22.2% for the average short-circuit current density. This resulted in a relative improvement of 22.5% for the average power conversion efficiency. Characterization by surface photovoltage and photoluminescence provides evidence that the implemented silver nanoparticles can enhance the charge separation and the trapping of electrons into the TiO 2 layer at the CH 3 NH 3 PbI 3 /TiO 2 interface. The application of these silver nanoparticles therefore has promise to enhance the ultra-thin perovskite solar cells.

In recent years, BiVO4 has been optimized as a photoanode material to produce photocurrent densities close to its theoretical maximum under AM1.5 solar illumination. Its performance is, therefore, limited by its 2.4 eV bandgap. Herein, nitrogen is incorporated into BiVO4 to shift the valence band position to higher energies and thereby decreases the bandgap. Two different approaches are investigated: modification of the precursors for the spray pyrolysis recipe and post‐deposition nitrogen ion implantation. Both methods result in a slight red shift of the BiVO4 bandgap and optical absorption onset. Although previous reports on N‐modified BiVO4 assumed individual nitrogen atoms to substitute for oxygen, X‐ray photoelectron spectroscopy on the samples reveals the presence of molecular nitrogen (i.e., N2). Density functional theory calculations confirm the thermodynamic stability of the incorporation and reveal that N2 coordinates to two vanadium atoms in a bridging configuration. Unfortunately, nitrogen incorporation also results in the formation of a localized state of ≈0.1 eV below the conduction band minimum of BiVO4, which suppresses the photoactivity at longer wavelengths. These findings provide important new insights on the nature of nitrogen incorporation into BiVO4 and illustrate the need to find alternative lower‐bandgap absorber materials for photoelectrochemical energy conversion applications.